In the art of large scale integrated (LSI) devices, the information packing density is constantly increasing. However, with the increasing packing density, certain problems or potential problems may present themselves.
It has been shown that the generation of thermally induced internal microdefects in semiconductors is strongly affected by the type and concentration of dopant, and that oxygen precipitation is drastically suppressed in heavily doped n-type (n+) or p-type (p+) silicon wafers. In this respect, it has been deemed important to measure the interstitial oxygen (O.sub.i) non-destructively in heavily doped wafers as well as heat-treated ones in order to understand the formation and behavior of thermally induced microdefects in the wafers.
However, the measurement of interstitial oxygen in silicon materials is not always a straightforward and easily accomplished goal. More specifically, many CMOS processes use epitaxial structures of a lightly doped layer of silicon on a heavily doped substrate to avoid latch-up problems. Typically, epitaxial structures are either n/n+ or p/p+.
Although optical transmittance measurements can be employed to measure interstitial oxygen under certain circumstances in some semiconductors, the interstitial oxygen (O.sub.i) of the n+ or p+ substrates, if the substrate is standard (SEMI) thickness, is not easily measured by optical transmittance for resistivities between 0.1 Ohm-cm and 0.02 Ohm-cm and cannot be measured for resistivities less than 0.02 Ohm-cm due to the optical interference of free carrier optical absorption. O.sub.i is measured at 1107 cm.sup.-1 (9 micrometers) where the localized Si--O bond stretching vibration occurs. Free carrier absorption interference creates problems for both the consumer and the silicon supplier when specifying O.sub.i concentration for n+ or p+ silicon. Furthermore, the inability to measure interstitial oxygen has led to contradictory results in precipitation studies of n+ silicon and to confusion regarding the effects of dopant type and concentration on the SiO.sub.x precipitation in n+ or p+ silicon.
To ensure that the O.sub.i meets the specification of the consumer, most silicon suppliers will grow a dummy ingot of lightly doped silicon (having a resistivity of greater than 1.0 Ohm-cm). Lightly doped silicon does not have the free carrier optical absorption interference, and the O.sub.i can be easily measured. If the O.sub.i of the dummy ingot meets specifications, then the dopant level is increased to meet the consumer resistivity specification, and a new ingot is grown under the same conditions as the dummy ingot. An assumption is made that the O.sub.i will not drastically change during the growth of the new ingot. However, this assumption is not always reliable. It would be desirable, therefore, to have a method of measuring O.sub.i that is reliable for silicon wafers that are heavily doped (having dopant concentrations greater than 10.sup.17 /cm.sup.3).
Other methods are known which include the determination of interstitial oxygen implicitly but not explicitly. These methods are for the determination of total oxygen, which includes both interstitial oxygen and substitutional oxygen, in silicon substrates. These methods for determining total oxygen include: secondary ion mass spectrometry (SIMS); vacuum fusion gas analysis; photoactivation analysis combined with gas fusion analysis; electron irradiation; positron annihilation; charged particle analysis; and Fourier transform infra-red spectroscopy (FTIR). The methods that provide total oxygen, as opposed to interstitial oxygen, are obviously not used for interstitial oxygen determinations. Furthermore, the techniques of SIMS, gas fusion, and irradiation are destructive of the samples, and further processing after analysis is impossible.
More specifically with respect to FTIR, FTIR can be used for the direct FTIR measurement of O.sub.i in n+ or p+ silicon. When FTIR is used for the direct FTIR measurement of O.sub.i in n+ or p+ silicon, some form of mechanically thinning of a wafer is always involved. Standard mechanical thinning practice requires that the wafer be sectioned into pieces of approximately 5.0 cm.sup.2 in order to maintain flatness and parallelism during the mechanical thinning and polishing processes. Sectioning into smaller pieces destroys substrate integrity and any further processing to produce a LSI or CMOS device is impossible.
A number of significant disadvantages are associated with the mechanical thinning of the silicon wafers. When a particular specimen is mechanically thinned for the FTIR testing to determine O.sub.i, too much damage may be done to the specimen which would increase the background noise level of the spectra thereby decreasing the accuracy of the measurement of the O.sub.i band. Thus, it would be desirable to provide a way of thinning a silicon wafer for FTIR testing which permits the tested specimen to be used in further processing steps to produce a LSI or CMOS device, or the like.
Furthermore, when a particular specimen is mechanically thinned for the FTIR testing, free carrier absorption interference can still be a problem. Subsequently, then, irradiation is carried out to neutralize the free carriers. It would be desirable, therefore, to provide a way of thinning a silicon wafer for FTIR testing in which free carrier optical absorption is significantly reduced, whereby there is no need for subsequent irradiation to the neutralize free carriers.
When a particular specimen is mechanically thinned for the FTIR testing, relatively small specimens are needed. However, for many LSI and CMOS devices, the entire wafer of silicon is employed. It is difficult to handle the small pieces of silicon, and it is difficult to subject the small pieces of silicon to mechanical thinning. It would be desirable, therefore, to have a way to prepare silicon samples for FTIR testing for interstitial oxygen without the need to handle and mechanically thin relatively small pieces of silicon.
Accordingly, it is an object of the present invention to provide a method for forming porous semiconductor material on semiconductor substrates.
Another object is to provide a method for selectively thinning semiconductor devices.
Another object is to provide a method for thinning complete integrated circuit chips for application in multichip packaging technology.
Another object is to provide a method for forming insulated electrical path through a thick conductive silicon substrate.
Another object is to provide a method for forming holes in integrated circuit chips, which will serve to align said chips into a three-dimensional package.
Another object is to form vias or holes through a conductive silicon substrate for making electrical connections the front and the back of the substrate.
Another object is to decorate structural grown-in or process induced defects on a silicon wafer.
Another object is to provide a method for preparing silicon samples that permits reliable measurement of interstitial oxygen (O.sub.i) in silicon wafers that are heavily doped.
Another object is to provide a method of selective thinning a silicon wafer for FTIR testing which maintains the original dimensions of the tested specimen so further processing steps to produce an LSI or CMOS silicon-based device, or the like, can be carried out.
Another object is to provide a method for determination of interstitial oxygen in silicon that does not require the making and using of dummy ingots.
Another object is to provide a method of thinning a silicon wafer for FTIR testing in which free carrier absorption is significantly reduced, whereby there is no need for subsequent irradiation to the neutralize free carriers.
Another object is to provide a method to conduct FTIR testing for interstitial oxygen without the need to handle and mechanically thin relatively small pieces of silicon.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.